The invention relates generally to a reaction tube system of a steam reformer for the indirect heating of cracking feedstock, such as the catalytic cracking of hydrocarbons, and more particularly to a plurality of reaction tubes and headers, the reaction tubes penetrating through the bottom of the steam reformer and being extended to the associated header by low-alloy tube sections attached to the reaction tubes by circumferential welds. The tube sections located outside the steam reformer are provided with internal insulation. These steam reformers are more generally referred to as tubular cracking furnaces in which the cracking process takes place at high pressures of 15 bar and more and at high temperatures of 600.degree. C. and more.
West German Pat. No. 15 42 530 describes a tubular cracking furnace in which groups of reaction tubes are connected to separate headers. The headers are provided with internal insulation and are located beneath the cracking furnace. Outside the furnace bottom, the reaction tubes formed of high-grade material are extended to the headers and are provided with internal insulation from the top edge of the furnace bottom. The internal insulation permits using lower-grade material or even unalloyed material for the extension sections of the reaction tubes. The extension section between the reaction tube and the associated header may be attached by means of connecting flanges or by welding.
For the tube sections which are exposed to pressure and heat loads, it is imperative that a certain definite internal temperature profile be maintained to avoid overload. This is achieved by selecting the right material and thickness for the internal insulation. The sizing of the internal insulation is governed by the desire to reduce the heat transfer to the inside surface of the extension tube section as created by the design and by the rate of heat dissipation by the tube to the ambient air. This design is based on defined internal and external temperatures and heat transfer coefficients.
Deterioration of the system by corrosion of the welds joining high-alloy tube sections and low-alloy or unalloyed tube sections was a prior art problem that had to be eliminated.